Engine cold-start emissions released before an exhaust catalyst has been sufficiently warmed may lower vehicle exhaust quality. Accordingly, engine control systems may use various approaches to expedite attainment of an activation temperature (e.g., a light-off temperature) at the exhaust catalyst.
One example approach involves increasing exhaust gas temperatures by operating the engine rich to generate high levels of engine-out carbon monoxide (CO), hydrogen (H2), and hydrocarbons and at the same time pumping air (herein referred to as secondary air injection) into the exhaust manifold upstream of the exhaust catalyst. The air pumped into the exhaust manifold may react with the exhaust gases generating an exothermic reaction. As a result, rapid catalyst heating may be achieved. However, the inventors herein have recognized potential issues with such an approach. During secondary air injection, high levels of enrichment are required to increase the percentage of combustible gases in the exhaust, which reduces engine efficiency and increases particulate emissions. Further, high levels of spark retard are required to increase the temperature of the exhaust. As a result, combustion stability and/or engine efficiency may be degraded.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: during an engine cold-start, adjusting an exhaust back pressure valve position based on a desired exhaust back pressure; determining an actual exhaust back pressure upstream of the valve; delivering a desired secondary air amount into an exhaust passage upstream of a catalyst based on the actual exhaust back pressure; and adjusting a fuel injection amount and a spark timing based on the actual back pressure.
As an example, during an engine cold start, while an engine temperature is below a threshold temperature, an exhaust back pressure valve coupled downstream of an exhaust catalyst may be adjusted to increase an exhaust back pressure upstream of the valve. At the same time, a secondary air pump may be operated to deliver secondary air into the exhaust manifold upstream of the catalyst. By using a post-catalyst exhaust back pressure valve, the time and temperature that a given mass of exhaust gas is in contact with catalyst parts are substantially increased, expediting catalyst activation. By utilizing secondary air injection in coordination with the exhaust back pressure, a more rapid heating of the exhaust catalyst may be achieved. Further, an amount of enrichment and an amount of spark retard that would be required when exhaust back pressure increase and secondary air injection are utilized coordinately for catalyst warm-up may be less than the fuel injection amount and the spark retard that would be required during secondary air injection alone. Consequently, by using less spark retard, efficiency and combustion stability may be improved, and by using less enrichment, particulate emissions and efficiency may be improved.
In this way, exhaust back pressure and secondary air injection can be advantageously used to expedite exhaust catalyst activation. The combination enables exhaust catalyst activation to be expedited without compromising combustion stability and efficiency. By rapidly heating the exhaust catalyst, cold-start exhaust emissions may be reduced.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.